An anime character who might almost fit the "arche-type", but is not actually a step-mother, would be Mikage Tsumura from Jubei-Chan. She bore a striking resemblance to Jiyu's mother, and her father fell for her momentarily even though he knew she was married. Of course, she was an enemy, but became vrienden after being defeated. I forget how much she tried to assume the role of "mother" while plotting a way to steal the lovely eyepatch, but I think she was manipulative.
A non-anime example that can to mind was Sedusa from the Power Puff Girls. Image:
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